It is known from an International Application published under No. WO 86/03833 to provide opto-electronic scale reading apparatus comprising a scale defined by grooves on a tape or the like, and which is illuminated by light from a primary light source. Light from the primary light source is reflected from the scale at the grooves thereby defining a plurality of secondary light sources which are viewed by a readhead. The readhead includes a spatial filter in the form of a pair of spaced gratings, whereby a pattern of interference fringes is produced on one of the gratings referred to as the analyser grating. The pattern of fringes is sampled by a detection system to provide signals indicative of relative movement between the scale and the readhead.
It is to be understood that the term light as used throughout this specification includes not only visible light but radiation in the infra-red and ultra-violet parts of the spectrum.
Scales have traditionally been made to the quality standards of diffraction gratings with some hundreds of grooves per millimeter length of the scale being put on the scale with great accuracy. A book entitled "Diffraction Gratings" by M C Hutley includes a chapter on manufacture of gratings and explains the difficulties of the manufacture of gratings to a useably quality, and the extreme lengths to which manufacturers must go to produce straight grooves with minimal surface roughness and no jagged edges.
When the scale is illuminated, however, diffraction of the light can take place at the scale grooves giving rise to destructive interference of much of the light, so that a significant proportion of the reflected light passes to the filter along the relatively small number of directions corresponding to the diffraction orders of the wavelength of the light used to illuminate the scale.
Because of this directionality, light from some points on the scale fails to contribute to the full fringe field at the analyser grating thereby resulting in incomplete spatial filtering.
It has been proposed in UK Patent No. 1,516,536 to illuminate the scale with a diffuse light source in an attempt to produce a more diffuse secondary light source at the grooves on the scale. This proposal, however, has little or no effect on the ordered diffraction pattern which comes off the scale.